acceptance tests for execresult() and returnszero()

See

* https://cfengine.com/dev/issues/2981
* https://cfengine.com/dev/issues/1438
* https://cfengine.com/dev/issues/3576
* https://cfengine.com/dev/issues/3577
* https://cfengine.com/dev/issues/3578
* and others linked from there

The changes to `evalfunction.c` are required to catch the executions of `execresult`.

Sample output with evalfunction.c changes from `cf-agent -K -I -f ./01_vars/02_functions/execresult_multiples.cf.sub`:

```
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: returnszero ran '/bin/env X=1 test -e CLASSONCE' successfully and it did not return zero
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: returnszero ran '/bin/env X=1 test -e CLASSONCE' successfully and it did not return zero
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: returnszero ran '/bin/env X=1 test -e CLASSONCE' successfully and it did not return zero
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
2013-09-30T10:17:16-0400     info: returnszero ran '/bin/env X=1 test -e CLASSONCE' successfully and it did not return zero
2013-09-30T10:17:16-0400     info: execresult ran '/bin/echo RETONCE' successfully
```